1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane Switching (IPS) mode LCD device to improve an aperture ratio.
2. Discussion of the Related Art
Among flat panel display devices, a Twisted Nematic (TN) mode LCD device is generally used because of its advantageous properties of high picture quality and low power consumption. However, the TN mode LCD device has the problem of a narrow viewing angle. This is because of the refractive anisotropy of a liquid crystal molecule. When a voltage is applied to an LCD panel including the liquid crystal molecule aligned in parallel to a substrate, the liquid crystal molecule is aligned in vertical to the substrate.
To overcome the problem of a narrow viewing angle, an In-Plane Switching (IPS) mode LCD device has been actively researched and studied, wherein the liquid crystal molecule is aligned in parallel to the substrate.
FIG. 1A is a plan view illustrating a unit pixel region of a related art IPS mode LCD device. FIG. 1B is a cross section view along I-I′ of FIG. 1A.
As shown in FIGS. 1A and 1B, a gate line 1 and a data line 3 are perpendicularly formed on a first transparent substrate 10, wherein the gate and data lines 1 and 3 define a unit pixel region. Substantially, as providing ‘n’ gate lines and ‘m’ data lines, the LCD device is provided with ‘n×m’ pixel regions. In the drawings, only one pixel region is shown for convenience of explanation.
At a crossing portion of the gate and data lines 1 and 3, there is a thin film transistor (TFT) including a gate electrode 1a, an active layer 5 and source and drain electrodes 2a and 2b. The gate electrode 1 a is connected with the gate line 1, and the source and drain electrodes 2a and 2b are connected with the data line 3. Also, a gate insulation layer 8 is interposed between the gate line 1 and the data line 3.
The pixel region is provided with a common line 4 which is parallel to the gate line 1. Also, at least one pair of electrodes, that is, a common electrode 6 and a pixel electrode 7 are formed in the pixel region to switch liquid crystal molecules, wherein the common and pixel electrodes 6 and 7 are formed in parallel to the data line 3. The common electrode 6 is formed at the same time as the gate line 1, and is connected to the common line 4. Also, the pixel electrode 7 is formed at the same time as the source and drain electrodes 2a and 2b, and is connected with the drain electrode 2b of the thin film transistor 9. Then, a passivation layer 11 is formed on an entire surface of the first substrate 10 including the source and drain electrodes 2a and 2b. Also, a pixel electrode line 14 is overlapped with the common line 4, wherein the pixel electrode line 14 is connected with the pixel electrodes 7, to thereby form a storage capacitor (Cst) with an insulation layer 8 therebetween.
Also, a second substrate 20 includes a black matrix 21 which prevents light from leaking on portions corresponding to the thin film transistor 9, the gate line 1 and the data line; and a color filter 23 which represents various colors. In addition, alignment layers 12a and 12b are formed on facing surfaces of the first and second substrates 10 and 20, thereby determining an initial alignment direction of the liquid crystal molecules.
A liquid crystal layer 13 is formed between the first and second substrates 10 and 20, wherein the liquid crystal layer 13 is driven by a voltage applied to the common and pixel electrodes 6 and 7. That is, the liquid crystal layer 13 controls the light transmittance based on the voltage applied.
If a voltage is not applied to the above-mentioned IPS mode LCD device, the liquid crystal molecules of the liquid crystal layer 13 are aligned along the direction of the alignment layer formed on the facing surfaces of the first and second substrates 10 and 20. However, a the voltage is applied between the common and pixel electrodes 6 and 7, the liquid crystal molecules are switched and aligned in parallel to the substrate, that is, in parallel to the gate line 1. That is, if the voltage is applied between the common and pixel electrodes 6 and 7, an electric field occurs therebetween, whereby the liquid crystal molecules transmit the light based on the electric field and the voltage.
FIG. 2 is a cross section view explaining a driving principle of a related art IPS mode LCD device. In the related art IPS mode LCD device, the common electrode 6 and the pixel electrode 7 are formed on the same substrate, whereby the electric field occurs in parallel to the substrate, that is, the horizontal electric field. Thus, because the liquid crystal molecules are aligned along the horizontal electric field, the alignment of liquid crystal molecule is changed based on the voltage applied, thereby realizing a wide viewing angle.
However, the common and pixel electrodes 6 and 7 are formed in the pixel region of displaying images, whereby a luminance deteriorates due to a low aperture ratio. That is, as an interval between the common and pixel electrodes 6 and 7 increases, the luminance improves because of the increase of aperture ratio. As the interval between the common and pixel electrodes 6 and 7 decreases, the luminance is lowered due to the decrease of aperture ratio. As increasing the interval between the common and pixel electrodes so as to improve the aperture ratio, the high voltage is applied to drive the common and pixel electrodes.
Just above the common and pixel electrodes, there is a vertical electric field instead of the horizontal electric field. Thus, the liquid crystal molecules positioned above the two electrodes are aligned differently from those of the other portions having the horizontal electric field. Accordingly, since the liquid crystal molecules positioned above the common and pixel electrodes are not aligned by the horizontal electric field, it can not be used as the driving area of displaying the images.